Purified bovine fibrinogen in aqueous solution (1 mg/ml) was subjected to gamma-irradiation (0-100 kR) in the presence and absence of free radical scavengers. The absorption and emission spectra along with the data on amino acid composition indicated loss of tryptophan and tyrosine residues in the protein concomitant with the dose of radiation exposure. This could be correlated with the decrease in clottability of fibrinogen and the increase in clotting time. Sodium nitrate, an electron scavenger, did not exert protective action, either on these amino acids or on the biological activity of the protein as the substrate of thrombin. On the other hand, potassium iodide, a scavenger of OH· radicals preserved both of these to the extent of 90-95%. The results suggest that aromatic residues have a role in the clotting function of bovine fibrinogen and that gamma-radiation could serve as a probe to ascertain their involvement in the biological integrity of nonenzyme proteins.

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